1. Field of the Invention
The present invention relates to an electromechanical transducer and, more particularly, to a transducer type commonly known as a double piston transducer in which energy can be selectively radiated from opposite ends of the transducer device.
2. Description of the Related Art
A device commonly known as a double piston transducer is an electromechanical or electroacoustical transducer known in the prior art. In its simplest form the device consists merely of a thin piece of active material in contact with a radiating medium on both sides and which can be driven electrically to induce a planar motion therein. For example, a flat disk or ring made of piezoelectric ceramic (such as a lead zirconate titanate formulation) which has electrodes on its flat faces and is polarized in the direction normal to the flat surfaces may act as such a vibrator. This type of device is commonly operated at frequencies near its first longitudinal resonance frequency to achieve a higher output. To achieve a reasonably low resonance frequency and a well controlled response in a compact device, it is common practice to mass load the two sides of the active material with inactive material pieces.
An example of a prior art mass loaded double piston transducer is shown in FIG. 1(a). Piezoelectric material rings 1 are bonded to form a composite piezoelectric stack 2 and electrically wired in parallel so that when a voltage is applied between the electrical loads, all of the individual rings 1 expand or contract in unison along the longitudinal axis of the device. At each end of the ceramic stack are bonded identical head mass elements 3 each having an outer face 4 which is in contact with the radiating medium 5. A stress rod or pretension bolt 6 and an associated nut 7 are used to join the components and to provide a compressive bias stress to the stack 2 of active elements. For simplicity of explanation the electrodes which are positioned between the rings 1, and the insulating washers which are positioned at the ends of the stack 2 of rings are not shown. Such details are within the knowledge of the ordinarily skilled in the art.
The device of FIG. 1(a) may be used as either a generator or receiver of mechanical or acoustic energy and is normally operated in a frequency band approximately centered on its primary resonance frequency. In this frequency band the two head masses 3 move in opposite relative directions while the stack of active material 2 alternately expands and contracts along its length.
It will be recognized by those of ordinary skill in the art that the performance of the device in FIG. 1(a) can be approximated by the analogous behavior of the simplified electrical equivalent circuit of FIG. 1(b). In the circuit, the two inductors m.sub.h represent the two head masses, and the compliance of the ceramic stack is represented by the capacitor C. The two inductors m.sub.e1 and m.sub.e2 represent the effective mass contribution of the two ends of the ceramic stack 2. C.sub.0 is the electrical clamped capacitance of the ceramic stack 2, and .0.:1 is the transformation ratio of the electromechanical transformer representing the transduction property of the piezoelectric stack 2. The open boxes in the outer legs of the equivalent circuit represent the equivalent radiation impedance Z.sub.rad seen by the radiating faces of the transducer. The equivalent currents u.sub.1 and u.sub.2 in these impedances represent the velocities of the moving faces of the transducer.
Because of the symmetry of the device of FIG. 1(a) the energy radiated from its two ends is equal. In the equivalent circuit representation, this symmetry makes it evident that the equivalent currents u.sub.1 and u.sub.2 are equal. In an acoustic device this means that sound energy is radiated equally to the far field in both directions along the longitudinal axis of the device. In some applications this is advantageous. However, in other applications it is necessary to employ a device which can radiate in either direction along the longitudinal axis without significant radiation in the other direction. In these applications, the prior art double ended longitudinal vibrator is not acceptable.
In an application requiring unidirectional radiation, it is possible to employ two separate devices each having only a single radiating face in contact with the medium 5. Such an arrangement is shown in FIG. 2(a) where two identical longitudinal vibrator devices are mounted back to back. Each of these devices is similar to the double ended radiator except that it has only one head mass 3 in contact with the radiating medium. The second head mass is replaced by a tail mass 8 which is free to vibrate. The electrical equivalent circuits for this pair of devices are shown in FIG. 2(b). Each of the two circuits is similar to the one circuit of FIG. 1(a) except for the replacement of one of the two equivalent head mass inductors m.sub.h by the equivalent inductance of the tail mass m.sub.t, and the elimination of the radiation impedance Z.sub.rad in series with this equivalent tail inductance.
The two transducers in this arrangement are separately driven, and each radiates in only one direction along their common longitudinal axis. In the equivalent circuit representation, it is evident that the equivalent currents u.sub.1 and u.sub.2 in the radiation impedances are completely independent. The two transducer arrangement is suitable for use in situations requiring unidirectional radiation. A disadvantage of this arrangement is that it is larger, heavier, more complex and more expensive than a single device which could fill the same function.